1. Field
Embodiments of the present invention relate to systems and methods for eliminating emissions from an air classification device. More particularly, embodiments of the present invention relate to an air classification device having a substantially closed loop system that prevents the emission of polluted air from the device.
2. Related Art
A well known system to separate solids is an air classification device, which employs an upward air stream to separate the solids by density, shape, and weight. Air classification devices are used in numerous applications, such as grain cleaning, de-dusting of plastic pellets, and separation of solids for recycling. For example, air classification devices are often used to separate solids resulting from the shredding of automobiles, household appliances, and other machinery comprising various different types of material.
Air classification devices are a continuous process, wherein the solids to be separated are fed into and out of the device via conveyors. The air classification device separates lighter solids from heavier solids. Depending on the solids to be separated and the overall air pressure of the air classification device, a solid considered “light” for one device may be considered “heavy” for another device. Air classification devices are commonly used to separate light solids, such as carpet, seat covering, some plastics, tire cords, insulation, and road dust and dirt, from heavier materials.
In operation, the air classification device supplies an upward-moving air stream through an air classifier chamber. Concurrent with upward-moving air being supplied to the chamber, solids to be separated are provided at a general upper end of the chamber and allowed to fall through the chamber via gravity. For those solids that are light enough to be carried by the upward-moving air, the air stream and the light solids are then transported to a secondary separator, such as a cyclone separator. The light solids are then further separated, so that a substantial majority of the solids is removed from the air stream and deposited in a hopper connected to the cyclone separator. The air stream is circulated back into the air classification device to continue the process of providing an upward-moving air stream to the air classifier chamber.
Although an air classification device is theoretically a “closed loop” system, high velocity air currents inside the device can result in additional air, not required for the process, being drawn in at one point, such as an inlet, and escaping at another, such as an outlet. The escaping air is commonly polluted, which results in undesired emissions at either or both of the inlet or outlet of the air classifier.
One method of preventing the undesired emissions is to “bleed off” a portion of the air used in the classification process to maintain a negative pressure inside the air classifier. This negative pressure induces air to move into the classifier at the inlet and the outlet, which contains the emissions inside the air classifier.
A disadvantage of bleeding off the air is that it will contain many airborne pollutants. Therefore, to meet the air quality of many regulatory agencies, the air must be directed to a filter, such as a baghouse style dust collector, where the air is filtered before being exhausted to atmosphere. The exhaust is treated as an emission point and must be monitored and regulated by local air quality authorities.
Another method of preventing the undesired emissions is the use of mechanical dampers to attempt to control air from entering the air classifier. The dampers are commonly a counterweight designed to ride or float across the varying bed of solids fed into and out of the respective inlet and outlet of the air classifier. The dampers minimize the area that any air can pass through the inlet and the outlet.
A disadvantage of using mechanical dampers to limit the area in which air can enter the air classifier is that the dampers are at least partially in the array of solids being fed into and out of the air classifier. Additionally, the dampers are subject to physical damage from the varying solids transported in and out of the air classifier on in-feed and out-feed conveyors. If one of the dampers becomes bent or broken, the damper becomes ineffective, and emissions will be allowed to escape from the air classifier, or the conveyors may become blocked, which results in unscheduled down time of the air classifier. Additionally, the dampers require regular maintenance and repair. Further, maintenance of the dampers is expensive. Without proper maintenance, the dampers result in undesired emissions. Finally, and most notably, the solids on the in-feed and out-feed conveyors comprise numerous irregular shapes. The dampers float over the solids and thus, provide a poor seal, such that emissions leak out of the spaces between the damper and the irregular shapes of the solids being conveyed.